Christopher is 2 times as old as Stephanie. 28 years ago, Christopher was 6 times as old as Stephanie. How old is Christopher now?
Solution: We can use the given information to write down two equations that describe the ages of Christopher and Stephanie. Let Christopher's current age be $c$ and Stephanie's current age be $s$ The information in the first sentence can be expressed in the following equation: $c = 2s$ 28 years ago, Christopher was $c - 28$ years old, and Stephanie was $s - 28$ years old. The information in the second sentence can be expressed in the following equation: $c - 28 = 6(s - 28)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to solve our first equation for $s$ and substitute it into our second equation. Solving our first equation for $s$ , we get: $s = c / 2$ . Substituting this into our second equation, we get: $c - 28 = 6($ $(c / 2)$ $- 28)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $c - 28 = 3 c - 168$ Solving for $c$ , we get: $2 c = 140$ $c = \dfrac{1}{2} \cdot 140 = 70$.